An important consideration in the design of gear systems is the minimization of friction between gear components. By minimizing friction between gear components, the efficiency of a gear system is increased. For example, in a gear system that is used for the transmission of power, transmission loss due to friction within the system is reduced when friction within the system is reduced. Further, by minimizing friction between gear components, the longevity of a gear system is increased. That is, by reducing inter-component friction in the gear system the rate of frictional wear on the components is reduced, thereby increasing the amount of time the system can be operated before it fails.
A common gear system of the prior art includes two or more gears having a circular body. Each gear includes a plurality of “teeth” along the periphery of its circular body. The teeth of the two gears intermesh such that force can be transmitted from one of the gears to the other through the intermeshing teeth. Thus, if a torque is applied to one of the gears causing the gear to rotate, the gear's teeth will exert a force on the teeth of the other gear, causing the other gear to rotate. The sliding of the respective sets of teeth against each other is a source of gear system friction.
One way in which designers have reduced friction between components of a gear system is by substituting rotatable pins for gear teeth. FIG. 1 is an isometric view of a prior gear system in which rotatable pins have been used in lieu of teeth. As can be seen from the figure, a multiple of rotatable pins 5 are positioned along the periphery of a wheel 10 and engage a worm screw 15. The pins are arranged in a single “row” along the circumference of the wheel. The worm screw has an hourglass shape and has a spiral grove 20 cut into its surface. The pins engage the worm screw by moving through the spiral groove.
The gear system of FIG. 1 is typically used to transmit power from a drive shaft 25 to an axle 30. More specifically, as a torque is applied to shaft 25 in the direction shown by arrow 35, the groove exerts a force on the pins it engages, causing the wheel to rotate in the direction shown by arrow 40. Bearings 45a and 45b support the wheel while allowing it to rotate.
As the pins 5 rotate through the groove they are free to turn about their longitudinal axes by virtue of bearings 50. For example, as pin 7 moves through the groove it rotates in the direction shown by arrow 55. Since the pins are free to rotate about their longitudinal axes, the friction between the pins and the walls of the groove is reduced. That is, since the pins can rotate about their longitudinal axes they can rotate about the walls of the groove. Whereas, if the pins could not rotate about their longitudinal axes they would have to slide against the walls of the groove.
While the gear system of FIG. 1 realizes the advantage of substituting rotatable pins for fixed teeth, it has several drawbacks. Three of the problems associated with the system of FIG. 1 are referred to as “pin slip,” “skid starting” and “wheel misalignment.”
The problem of “pin slip” is caused by the centrifugal force acting on pins 5 as wheel 10 rotates. FIG. 2 is a plan view in profile of some of the elements of the gear system of FIG. 1. In particular, FIG. 2 shows pins 5, bearings 50 and worm screw 15. Also shown are spiral groove 20, drive shaft 25 and a plurality of internal bearings 60. The internal bearings are internal to wheel 10 and help support the pins.
As can be seen from FIG. 2, rotation of the worm screw in the direction shown by arrow 65 causes movement of the pins 5 in the rotary direction shown by arrows 70. Such movement gives rise to a centrifugal force on the pins which is illustrated by arrows 75. The centrifugal force urges the pins radially outward from the center of the wheel, and if the pins are not protected against outward radial movement, the force moves the pins radially outward. It is the radially outward movement of the pins due to centrifugal force that is referred to as “pin slip”.
FIG. 3A illustrates the effect of pin slip. The figure shows a slipped pin entering the spiral groove of the worm screw. As can be seen from FIG. 3A, the pin does not enter spiral groove 20 smoothly. Indeed, as the pin moves into position to enter the groove, it could strike the base of the groove. The harsh entrance of the pin into the groove, and any attendant roughness in the remainder of the pin's travel through the groove, reduces the gear system's efficiency and increases the rate of wear and tear.
FIG. 3B is provided as a contrast to FIG. 3A. FIG. 3B shows how a pin that has not slipped enters the spiral groove of the worm screw.
The problem of “skid starting” is explained with reference to FIG. 1. Skid starting is related to the initiation of the rotation shown by arrow 55. More specifically, as pin 7 exits spiral groove 20 there is no force on the pin to maintain its rotation about its longitudinal axis, thus the rotation of the pin will decrease or stop during the time that it is not within the spiral groove. Thus, as the pin travels about the center of wheel 10 and once again enters groove 20, the groove exerts a torque about the pin's longitudinal axis. The torque is exerted on the pin by the wall of the groove (see e.g. FIG. 3B). The initiation of torque between the groove wall and the pin causes the pin to skid rather than roll into the groove, resulting in a roughness in the system's operation, which decreases efficiency and longevity.
The problem of “wheel misalignment” is explained with reference to FIG. 1. Referring to FIG. 1, rotation of the worm screw in the direction of arrow 35 applies a force on pins 5 in the direction shown by arrow 80. More specifically, during rotation of the worm screw in direction 35, the force exerted on pins 5 by groove 20 can be described as including two components, a first component which urges the pins to move in the direction shown by arrow 40 and a second component which urges the pins to move in the direction of arrow 80. Both of the component forces are transmitted to wheel 10, the first component urging the wheel to turn in direction 40 and the second component urging the top of the wheel to move in direction 80. Any movement of the wheel in direction 80 is a source of wheel misalignment. That is, any movement of the wheel in the direction 80 changes the path of the pins relative to the worm screw. The change in path takes the pins off of their intended path and gives rise to roughness and/or inefficiency of operation.
It is important to note that in FIG. 1 it is typical for the forces associated with arrow 80 to exert force on the top of the wheel so as to urge the top of the wheel to move in the direction of arrow 80. But for the fixed center axis of the wheel, this force would cause the bottom of the wheel to move in the opposite direction, as shown by arrow 85. In actual operation of the gear system over extended periods at high velocities, the force at the top of the wheel tends to exceed the restraint of the wheel axis; thereby causing the wheel axis to become deflected, which results in wheel misalignment. For example, if in normal operation the wheel's axis is aligned with the horizontal direction in FIG. 1, wheel misalignment could deflect the axis so that there is some angle between the axis and the horizontal direction.
It is submitted that the dynamic instabilities of pin slip, skid starting, and wheel misalignment have frustrated prior attempts to successfully commercialize rotatable pin type worm-gear assemblies.